Page:Proceedings of the Royal Society of London Vol 69.djvu/343

Rh and we have little doubt on the point, that the true eutectic angle for all alloys from B to D is at C', and that the apparent depression of X is a retardation due to the difficulty experienced by the a in crystalli- sing from the solid solution without a nucleus of its own kind. Thus, in the region XDoD' the alloys are a complex of ft and D', while below XD2 they form a complex of a and D'. As we have said before, we feel no doubt that this D' is the compound CojSn.

The alloy Sn^o, although it undergoes a well-marked exothermic transformation at the D' temperature, remains substantially uniform. The fact that it has recrystallised is, however, shown by minute traces of the C' eutectic, visible between the large crystals of CiitSn. It may be that the chemical compound Cu 4 Sn does not exist above the temperature D'.

5. Ttie DE Alloys, containing from 20 to 25 atomic per cents, of Tin. Between the liquidus and solidus these alloys contain primary combs of y. On the solidus these combs fill the alloy, and just below it they form a uniform solid solution, but it is very difficult in this region to avoid a commencement of the transformation proper to the D'E' curve. However, our chilled alloys afford abundant evidence that the normal state of alloys between de and D'E' is that of a uniform solid solution. When the temperature falls to a point on the curve D'E', long, straight, very uniform tin-rich bars separate out of the solid solution. These are very slender and scanty near D', but become massive and abundant as we approach E', and at that point fill the whole alloy. These bars are really plates of E', seen more or less edgeways, and their appearance of greater or less breadth is partly due to -their inclination. These plates, the first appearance of the E' phase, must be either pure Cu 3 Sn or mixed crystals of Cu 3 Sn and Cu 4 Sn ; we are not at present able to decide this point. Thus, in the area D'E"E 2 F'E', the alloys are a complex of E' + y. But Roberts-Austen and Stansfield have proved that these DE alloys show, when they fall to the temperature D', an evolution of heat. This must be due to the conversion of the residual y into D', so that below D'E" the alloys form the complex D' + E'.

6. The EF Alloys, containing from 25 to about 27*5 atomic per cents, of Tin. These go through the same stages of y + liq., then pure y, then y + E', but at the temperature G the residual y breaks up into E' and the G liquid.

It may be noted here that the triangular area Ixf forms an island of typical uniform solid solution, which could only have been discovered by the examination of chilled alloys.

7. The FG Alloys, containing from 27'5 to 42 atomic per cents, of Tin. These alloys, like the preceding, begin by forming the complex y + liquid ; their state, when the temperature G is reached, being y crystals of the / percentage and liquid of the G percentage. The isothermal transformation y crystals = E' + G liquid, now begins.